(1) Field of the Invention
The present invention relates to a continuous process for the removal of hydrogen sulfide from a gaseous stream; the conversion of sulfide to polysulfide in an electrolytic cell; and the production of sulfur in a heating zone.
(2) Description of the Prior Art
The removal of H.sub.2 S from a gas stream is a problem that has long confronted and challenged workers in many diverse industries. One example is in the natural gas industry where the H.sub.2 S content of certain gas streams recovered from natural gas deposits in many areas of the world is often too high for commercial acceptance. Another example is in the manufactured gas industry or the coke-making industry where coal gas containing unacceptable amounts of H.sub.2 S is commonly produced by the descructive distillation of bituminous coal having a high sulfur content. Yet another example is found in the manufacture of water gas or synthesis gas where it is not unusual to produce gas streams containing H.sub.2 S by passing steam over a bed of incandescent coke or coal containing a minor amount of sulfur.
More frequently, this problem is encountered in the petroleum refining industry because the principal raw material used, crude oil, typically contains a minor amount of sulfur--principally in the form of organic sulfur compounds. During the course of the many processes to which the crude oil or fractions thereof are subjected, one or more gas streams containing H.sub.2 S are quite commonly produced. For example, in many cases one of the product streams from a hydrocarbon conversion process is a gas stream containing H.sub.2 S in admixture with hydrogen and/or with light normally gaseous hydrocarbons mainly, C.sub.1 -C.sub.3. As is well known in the art, the presence of H.sub.2 S in these refinery gas streams can cause a number of detrimental problems in subsequent processing steps such as: corrosion of process equipment, deterioration and deactivation of catalysts, undesired side reactions, increases in process pressure requirements, increase in gas compressor capacity, etc.
Regardless of the source of the gas stream containing H.sub.2 S, the problem of removing H.sub.2 S therefrom has been solved in a number of different ways which generally involve one or more of the following techniques: selective absorption with a wide variety of absorbents, adsorption by a suitable adsorbent, selective reaction with a reagent which produces an easily separable product, etc. The details of these techniques are well known to those skilled in the art. One old and well-known solution to this H.sub.2 S removal problem involves scrubbing the gas stream with an ammoniacal aqueous solution. For example, in Germany the perox process, which uses ammonia scrubbing, has been widely used for coal gas purification. Despite the considerable amount of effort that has been devoted to developing an acceptable solution to this problem involving scrubbing with an ammoniacal solution, the use of ammoniacal scrubbing has not been universally accepted in the gas treating art as the preferred method for removing H.sub.2 S from a gas stream primarily because of a number of operational difficulties associated with its implementation. One difficulty involves the relatively high partial pressure of ammonia at preferred scrubbing temperatures which generally requires that the scrubbing step be conducted with a relatively dilute ammonia solution or under relatively high pressure. The use of a dilute scrubbing solution in turn quite commonly forces the addition of a separate water wash step after the ammonia scrubbing step in order to remove ammonia from the treated gas stream. In addition, the use of dilute scrubbing solutions typically increases substantially the regeneration costs where the regeneration step is conducted at a considerably higher temperature than the scrubbing step, although some of this heat load can be recovered by a suitable heat exchanging procedure. Another difficulty is associated with the regeneration of the rich absorbent solution withdrawn from the H.sub.2 S-scrubbing step. In order to minimize the requirements of the scrubbing step for water and ammonia, it is necessary to remove sulfide from this rich absorbent. Several regeneration procedures have been proposed but they typically have involved the use of absorbent-soluble catalysts such as hydroquinone and have had problems such as contamination of the sulfur product with the catalyst, excessive formation of undesired by-products such as ammonium sulfate and thiosulfate and loss of scrubbing solution and catalyst during the periodic purges that are generally required to remove side products from the system. Other difficulties have been associated with the recovery of the elemental sulfur from the regeneration step. In some processes it has been customary to form a froth of sulfur in the absorbent regeneration vessel which then must be skimmed off and filtered. In short, it is clear that there are a significant number of technical problems associated with the prior art methods for removing H.sub.2 S from a gas stream by the method of scrubbing with an ammoniacal solution.
In U.S. Pat. No. 3,409,520, a process is disclosed for the removal of hydrogen sulfide from a hydrogen sulfide/hydrocarbon gas mixture using an electrolytic cell having a basic electrolyte. The base can have a cation selected from the group consisting of sodium, potassium, and ammonium ions. Free sulfur or polysulfide products are produced in the anode compartment of the electrolytic cell. In this process, the hydrogen sulfide present in a gaseous stream is fed directly to the porous anode of the electrolytic cell.
In U.S. Pat. No. 3,249,522, hydrogen sulfide contained in a gaseous stream is utilized as a feed gas in a fuel cell. The gaseous stream is fed directly to a porous anode of an electrochemical cell containing a basic electrolyte. The base can have a cation selected from the group consisting of sodium, potassium, and ammonium ions. Free sulfur is formed in the anode compartment of the cell. A sulfur solvent can be utilized to prevent clogging the pores of the porous anode with sulfur.
In U.S. Pat. No. 4,342,731, a method is dislosed for removing hydrogen sulfide from coke oven gas utilizing an ammonia scrubber and in U.S. Pat. No. 3,715,426, a continuous process is disclosed for scrubbing hydrogen sulfide from a gas stream and selectively producing elemental sulfur wherein the hydrogen sulfide is scrubbed from the gaseous stream utilizing an aqueous solution of ammonium hydroxide.